1. Field of the Invention
The present invention relates to a back-illuminated type photoelectric conversion device and a method of driving the same.
2. Description of the Prior Art
A conventional photoelectric conversion device and a driving method thereof will herein be explained by way of example. FIG. 1(a) is a sectional view of a unitary picture element for a CCD image sensor in which the conventional back-illuminated type photoelectric conversion devices are arranged in two-dimensional configuration. In this example, an infrared sensor made of a platinum silicide/silicon Schottky diode is used as a photodiode. The photodiode is formed by bringing platinum silicide film 1 into Schottky contact with P-type silicon substrate 2 and n guard ring 3 including n.sup.+ region 4 is formed on the periphery thereof for suppressing leak currents. The electric charge accumulated in the photodiode is transferred through a CCD adjacent thereto and outputted to the outside. The CCD is composed of n well 6, transfer electrode 10 and transfer gate electrode 9.
FIG. 1(b) illustrates a driving waveform applied to transfer gate electrode 9 for reading out the charge. A potential of high level during a time interval of t.sub.1 -t.sub.2 causes transfer gate electrode 9 to be driven into conduction and allows the potential of the photodiode to be reset, thereby concurrently effecting the reading out of the accumulated charge. Thereafter, the charge is further transferred by the CCD, but the explanation on the operation thereof is omitted. The electric charge is again read out at time t.sub.3, and a time interval of t.sub.3 -t.sub.2 provides a charge storage time. This reading operation is performed every one field or one frame of a sensor picture plane.
In this case, the infrared ray is incident from the rear surface, so that anti-reflection coating 12 for reducing reflection is formed on the rear surface in order to make the absorption of light by the photodiode maximum. Reflecting plate 7 is formed toward platinum silicide film 1 being separated by insulating film 8 in such a manner that the loop of the standing wave of the light may come on platinum silicide film 1. Ordinarily, this type of reflecting plate is made of a metal such as aluminum or the like. This sensor is generally used for detecting infrared rays having a wavelength of 3-5 .mu.m, so that the distance between the platinum silicide and the reflecting plate is so adjusted that the absorption peak occurs for a wavelength of about 4 .mu.m. Theoretically, this optical distance is defined by an expression of (2n+1)/4 wavelength (where, n is any integer above zero), and generally chosen to be approximately 1/4 wavelength in order to make the light absorption at the insulating film minimum. More concretely, as shown in the proceeding of SPIE, VOL. 1685, pages 2 through 19, the thickness of the insulating film is 7500 .ANG.ngstrom (hereinafter referred to as .ANG.)in the case of SiO.sub.2, and 5600 .ANG. in the case of SiO. The difference of the thickness between these two insulating films is due to the difference in the index of refraction. Like these, the optimum value of the distance between the platinum silicide and the reflecting plate can be determined based on the selection of insulating film. In this case, the layer between the platinum silicide and the reflecting plate serves as part of the photodiode capacitance.
FIG. 4 is a diagram showing the photodiode capacitance as a function of photodiode reset voltage, where the capacitance is represented by the number of electrons which can be read into the CCD when the distance between the platinum silicide and the reflecting plate is varied under the condition that the reflecting plate is grounded or not grounded. The insulating film used in this case is SiO.sub.2. It can be understood from the comparison between the results obtained when the thickness of the oxide film is close to the optimum value, 8000 .ANG. (represented by square symbols), and the results (a curve represented by X symbols in the lowermost part) obtained when the reflecting plate is not grounded, that the capacitance between the platinum silicide and the reflecting plate corresponds to a large component accounting for approximately half the photodiode capacitance. Since the background component resulting from a radiant light of 300.degree. K. or the like is large in the infrared sensor, it is required to increase the photodiode capacitance in order to expand its dynamic range. As described above, since the capacitance between the platinum silicide and the reflecting plate is a major component of the photodiode capacitance, the photodiode capacitance can be effectively increased by rendering the aforesaid capacitance large. However, it is known that there exists an optimum value for the distance between the platinum silicide and the reflecting plate in view of its optical properties, so that it is not possible to decrease the thickness of the insulating film in order to increase the photodiode capacitance. Upon driving of the conventional photoelectric conversion device shown in FIG. 1(a), the grounding of the reflecting plate and the application of pulse voltage are not performed unlike the present invention. When the reflecting plate is not grounded, the capacitance between the platinum silicide and the reflecting plate is not available because the potential in the reflecting plate becomes equal to that of the photodiode.